Method of producing phosphoric acid salt

ABSTRACT

A method of preparing a phosphoric acid salt which includes at least one attack on phosphate ore by an aqueous solution of hydrochloric acid, with the formation of an attack liquor, a first separation, in the attack liquor, between an insoluble solid phase and an aqueous phase, a neutralization of the separated aqueous phase by the addition of a calcium compound in order to form, with phosphate ions contained in this aqueous phase, a calcium phosphate insoluble in water, which precipitates, and a second separation, in the neutralized aqueous phase, between a liquid phase and a precipitated solid phase based on the calcium phosphate insoluble in water, wherein the attack on the phosphate ore includes dissolving the phosphate in the ore, the attack liquor containing this phosphate in the form of phosphate ions, and the solid phase separated from the attack liquor contains impurities and the aqueous phase separated from the attack liquor contains the phosphate ions thereof, chloride ions and calcium ions, this aqueous phase being subjected to the neutralization and second-separation steps.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.10/519,562, filed Dec. 28, 2004, which was a national stage applicationof PCT/BE2003/000111, filed 26 Jun. 2003.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing phosphoric acid,in particular pure phosphoric acid, comprising

-   -   at least one phosphate ore attack with a first aqueous solution        of hydrochloric acid, with the formation of an attack liquor,    -   a first separation, in the attack liquor, between an insoluble        solid phase containing impurities and a separate aqueous phase        comprising in solution phosphate ions, chloride ions and calcium        ions,    -   an extraction of an aqueous solution containing phosphate ions,        chloride ions and calcium ions by an organic extraction agent,        in order to form an aqueous extraction phase comprising chlorine        ions and calcium ions and an organic extraction phase containing        phosphoric acid, and    -   a re-extraction of the organic extraction phase by an aqueous        re-extraction agent, in order to isolate an aqueous        re-extraction phase containing phosphate ions, as well as,        possibly,    -   a concentration of the aqueous re-extraction phase in order to        form an aqueous solution of pure phosphoric acid.

Methods of this type have been known for a long time, involving ahydrochloric attack on the ore and a liquid-liquid extraction of theliquid phase resulting from a separation, from the attack liquor, of theinsoluble materials (see for example the patents U.S. Pat. No.3,304,157, GB-1051521 and ES-2013211, as well as the article I.M.I.Staff Report, Development and implementation of solvent extractionprocesses in the chemical process industries, in Proc. Int. Solv. Extr.Conference, ISEC'71, The Hague, Apr. 19-23, 1971, Paper No 94).

These methods have the drawback of generally making use, for the attack,of a concentrated solution of HCl, which may be as much as 20% and even30% by weight. The ore to be used must be of good quality, that is tosay with a high P₂O₅ content, and a fine grinding of the ore isgenerally required, which increases costs. During the attack a thermalshock is obtained, due not only to the exothermal nature of thereaction, but also to the dissolution energy released, and the insolublematerials are therefore difficult to separate, since the liquor obtainedis viscous and loaded with original organic materials of the ore. Giventhe high temperature and the concentration of the HCl solution,significant problems of corrosion arise.

Another major drawback of these methods lies in the extraction, by anorganic solvent, of an aqueous phase which has a relatively low P₂O₅content. It contains at a maximum 5% to 6% of this by weight. Extractionis therefore difficult and with a poor yield, and the step ofconcentrating the aqueous solution of phosphoric acid obtained afterre-extraction consumes a great deal of energy.

Finally, in the aqueous phase issuing from the extraction and containingin solution CaCl₂ and hydrochloric acid, it is necessary to recover thetraces of organic solvent entrained. However, this aqueous phase is verybulky and this step also consumes a great deal of energy, in particularif to do this a steam entrainment is applied.

A hydrochloric attack method is also known in which the ore is subjectedto a first limited attack with dilute hydrochloric acid. The solidfraction, thus enriched with P₂O₅, is then subjected to a second attackwith hydrochloric acid and then concentrated, and then to aliquid-liquid extraction in order to produce phosphoric acid (see U.S.Pat. No. 3,988,420).

Other methods of producing phosphoric acid are of course known, forexample the thermal method based on the combustion of phosphorus, or wetmethods also based on an attack on the phosphate ores with an acid,generally sulphuric acid, the latter method giving rise to a highproduction of gypsum, as a by-product.

The aim of the present invention is to develop an improved method ofproducing phosphoric acid and/or a salt thereof, by wet method, byattack using hydrochloric acid. This method must make it possible toavoid the aforementioned drawbacks, in particular a fine grinding orcalcination of the extracted ore, without having any particularrequirement on the quality of the ore, and it must propose an efficientliquid-liquid extraction giving rise to a concentrated pure phosphoricacid solution, without harming the environment.

SUMMARY OF THE INVENTION

These problems are resolved according to the invention by a method asdescribed at the start, which also comprises

-   -   a neutralization of the said aqueous phase separated from the        attack liquor by the addition of a calcium compound in order to        form with the said phosphate ions a calcium phosphate insoluble        in water, which precipitates,    -   a second separation, in the said neutralised aqueous phase,        between an aqueous phase comprising in solution calcium ions and        chloride ions and a precipitated solid phase based on the said        calcium phosphate insoluble in water, and    -   a solubilization of at least part of the said precipitated solid        phase separated, in a second aqueous solution of hydrochloric        acid, with the formation in the said aqueous solution containing        phosphate ions, chloride ions and calcium ions to be extracted        by means of an organic extraction agent.

This method has the advantage of providing an attack on an ore which canhave a moderate P₂O₅ concentration by means of a fraction ofhydrochloric acid which can be dilute, a separation of a solid phaseenriched with P₂O₅ with the non-attacked materials insoluble in waterremoved and a solubilization of this solid phase enriched with P₂O₅ by asecond more concentrated fraction of hydrochloric acid. As thehydrochloric attack takes place in a dilute and therefore non-viscousenvironment, the separation of the insoluble materials is simple andrapid, there is no release of heat during the attack, whichadvantageously occurs at room temperature, and the problems of corrosionby the hydrochloric acid are largely avoided.

Moreover, the liquid-liquid extraction is carried out in a solubilisedliquid phase with a high P₂O₅ content and its efficiency can thereforebe greatly increased thereby. The number of extraction stages forexample can be appreciably reduced and any final concentration requiresa lesser consumption of energy, compared with the prior art cited.

In the method according to the invention, the quality of the ore haslittle importance. Its P₂O₅ content can for example be from 15% to 38%by weight, without any problem, and ores with a favourable cost cantherefore be used. It is possible for example to envisage an ore having,in % by weight, 20-38% P₂O₅, 34-40% Ca and 5 to 10% impurities, with aCa/P ratio of 1.5 to 2.4.

Through the subsequent neutralization, a precipitate is produced whichcan be termed “enriched” with P₂O₅ and purified with respect to the ore,since its P₂O₅ content can be from 40% to 50% by weight.

The ore does not have to be finely ground and it can have a grain sizeof around 150-500 μm.

The hydrochloric acid used in the attack is preferably in the dilutestate. It advantageously has a maximum concentration in water of 10% byweight, advantageously from 3% to 6%, preferably less than or equal to5%. This attack, which is not very aggressive, in a non-viscous medium,is thereby very selective, that is to say it solubilises the P₂O₅preferentially and completely and few impurities. The separation of theinsoluble and unattacked substances takes place in a simple and completemanner, which therefore makes it possible then to obtain an insolublephosphate salt “enriched with P₂O₅” and profoundly devoid of impurities.In the attack liquor, the normalized molar ratio between HCl and Ca ispreferably between 0.6 and 1.3, advantageously between 0.7 and 1.2,preferably of 0.8.

During the attack, a reaction takes place between HCl and the orecomponents containing Ca. For reacting with 1 mole of Ca 2 moles of HClare necessary to obtain a stoechiometric reaction and a component CaCl₂is formed such that the molar ratio Cl/Ca=2.

By “normalized” molar ratio between HCl and Ca according to theinvention it is necessary to understand that the above indicatedstoechiometric molar ratio HCl/Ca=2 has been normalized to 1.

According to this normalized terminology, a HCl/Ca ratio=1 means that 2moles of HCl are added to 1 mole of Ca. When a HCl/Ca ratio is equal forexample to 0.8, it is directly clear that 80% of the stoechiometricamount of HCl is added (chemically speaking in this case 1.6 moles HClare added to 1 mole of Ca).

The method according to the invention therefore allows the use of orewith a low P₂O₅ content, for example containing Fe and Mg, and procuresfor the operator great flexibility in the choice of the ore on themarket. It permits him the use of dilute hydrochloric acid, which alsohas a favourable cost, and makes it possible to avoid the problems ofthermal shock and viscosity at the time of attack.

The separation of the attack liquor can be effected by an appropriatemeans, for example by filtration, decantation or a similar process.

The aqueous solution thus obtained contains in solution calcium chlorideCaCl₂, phosphoric acid and water-soluble calcium dihydrogenophosphate(MCP), and is brought to neutralization.

In the neutralization step, there is introduced into this aqueoussolution a calcium compound, for example calcium hydroxide, calciumoxide, or a water-soluble calcium salt, such as calcium carbonate.

Through the increase in the pH, calcium monohydrogenophosphate (DCP) isformed, insoluble in water, which precipitates.

This precipitate can be separated, for example by filtration. Thefiltration cake can for example contain 40% to 50% P₂O₅, 25% to 28% Caand possibly small traces of impurities.

For the solubilization of the DCP, hydrochloric acid is used once again,but in a higher concentration, for example from 15% to 20% by weight. Aclear solution is then obtained, since the insoluble materials havealready been eliminated and the coefficient of extraction will be ableto be favourable since the P₂O₅ concentration is already relativelyhigh, around 8% to 15% by weight P₂O₅, preferably 10% to 13%, in thesolubilised solution.

During the extraction step it is possible to use any appropriate organicextraction agent, for example those recommended in the patents U.S. Pat.No. 3,304,157, GB-1051521, GB-1142719, FR-1427531 and FR-1396077, inparticular n-butanol.

According to one advantageous embodiment of the invention, the methodalso comprises, after the said extraction, a washing of the organicphase extracted by a fraction of the aqueous re-extraction phase, inorder to remove from the extracted organic phase a hydrochloric acidcontent and calcium chloride entrained by it and any impurities stillpresent. After re-extraction, an aqueous solution is thus obtainedhaving a P₂O₅ concentration of 15% to 25% by weight, which is anexcellent yield. Moreover, the residual HCl content may possibly be from1% to 5% by weight, which makes it possible to reduce the recycling ofHCl and the corrosion problems which are the consequence thereof.

According to an improved embodiment of the invention, the method alsocomprises a steam entrainment of traces of organic extraction agent fromthe aqueous extraction phase containing chloride ions and calcium ions.It is known how to recover the traces of organic extraction agent in theaqueous phase issuing from a liquid-liquid extraction. However, in themethod according to the invention, part of the CaCl₂ to be discharged asa by-product has already been isolated during the separation which tookplace after the precipitation of the DCP. There it was able to beeliminated, as it stood, without any particular purification step. Theaqueous solution of CaCl₂ coming from the extraction of the liquid phaseenriched with P₂O₅ according to the invention is therefore a volumewhich is appreciably reduced compared with the prior art and thequantities of steam necessary for entraining the organic extractionagent are appreciably lower.

According to an improved embodiment of the invention, the method alsocomprises a treatment of the aqueous phase issuing from the secondseparation and containing calcium ions and chloride ions in solution, byan aqueous solution of sulphuric acid with the formation of insolublecalcium sulphate, which precipitates, and an aqueous phase based onhydrochloric acid, an isolation of the calcium sulphate precipitate andan at least partial recycling of the aqueous phase based on hydrochloricacid in order to form the said first and/or second aqueous solution ofhydrochloric acid. The exploitation of the method according to theinvention can thus be envisaged on industrial sites which do not haveany hydrochloric acid. The recycled hydrochloric acid is of greaterpurity and therefore does not require frequent renewal.

As can be seen, the method according to the invention can be implementedin two successive modules, a first intended for the preparation of asolid phase based on DCP and a second intended for the production ofphosphoric acid at the start of this solid phase based on DCP. Sincethere is no flow of material connecting the two modules, it isabsolutely unnecessary for the steps performed in the second module toimmediately follow those performed in the first module. It is thereforepossible to conceive, for example, a drying of the solid phase based onDCP obtained after the precipitation and the separation of the DCP,storage of this dried solid phase and even transportation thereof to asite different from that where the steps of the first module areperformed.

The present invention can therefore cover a method in which at leastpart of the DCP obtained by precipitation is used for purposes otherthan that of serving as a base material in the second module. It ispossible for example to envisage the use of this phosphoric acid salt inthe field of fertilisers or the feeding of cattle.

The present invention covers the steps provided for in each of themodules separately.

Consequently the present invention also concerns a method of preparingphosphoric acid salt.

A method of preparing DCP is already known, comprising

-   -   at least one attack on phosphate ore by an aqueous solution of        hydrochloric acid, with the formation of an attack liquor,    -   a first separation, in the attack liquor, between an insoluble        solid phase and an aqueous phase,    -   a neutralization of the said separated aqueous phase by the        addition of a calcium compound in order to form, with phosphate        ions contained in this aqueous phase, a calcium phosphate        insoluble in water, which precipitates, and    -   a second separation, in the said neutralised aqueous phase,        between a liquid phase and a precipitated solid phase based on        the said calcium phosphate insoluble in water (see U.S. Pat. No.        3,988,420 already cited).

This method is in fact a method where the aqueous phase separated fromthe attack liquor contains as few phosphate ions as possible, thesebeing able to be, simultaneously with the production of phosphoric acid,recovered in the form of DCP.

A method of preparing DCP is already known, by an attack on the ore bymeans of a concentrated solution of hydrochloric acid (35%) with all thedrawbacks already cited above (see U.S. Pat. No. 1,470,663).

According to the invention, provision is made, unlike the methoddescribed above with regard to U.S. Pat. No. 3,988,420, for the attackon the phosphate ore to comprise a dissolving of the phosphate in theore, the attack liquor containing this phosphate in the form ofphosphate ions, and in that the solid phase separated from the attackliquor contains impurities and the aqueous phase separated from theattack liquor contains the said phosphate ions thereof, chloride ionsand calcium ions, this aqueous phase being subjected to the saidneutralization and second-separation steps.

The invention also relates to a method of producing phosphoric acid,comprising

-   -   a solubilization of a solid phase based on a calcium phosphate        insoluble in water in an aqueous solution of hydrochloric acid,        with the formation of a solubilised aqueous solution containing        phosphate ions, chloride ions and calcium ions,    -   an extraction of the aqueous solution solubilised by an organic        extraction agent, in order to form an aqueous extraction phase        containing chloride ions and calcium ions and an organic        extraction phase containing phosphoric acid, and    -   a re-extraction of the organic extraction phase by means of an        aqueous re-extraction agent, in order to isolate an aqueous        re-extraction phase containing phosphate ions, as well as,        possibly,    -   a concentration of the re-extraction aqueous phase in order to        form an aqueous solution of pure phosphoric acid.

The invention therefore also covers a production of phosphoric acid fromsolid commercially available DCP, and possibly manufactured by anymethod.

Other embodiments of the methods according to the invention areindicated in the claims given below.

Other details and particularities of the invention will emerge from thedescription given below non-limitingly of embodiments of the methodaccording to the invention, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts, in the form of a flow diagram, a module for purifyingand enriching the ore which can be used in an embodiment of the methodaccording to the invention, and

FIG. 2 depicts, in the form of a flow diagram, a phosphoric acidproduction module which can be used in an embodiment of the methodaccording to the invention.

DETAILED DESCRIPTION

As illustrated in FIG. 1, the phosphate ore extracted is fed at 1 into adigestion tank 2 where it is subjected to attack by a hydrochloric acidsolution of around 5% by weight, brought into this tank through a pipe3. Phosphate ore as extracted means that it is subjected neither tocalcination nor to fine grinding, in particular in the mines where theextracted ore is in powder form. If it is a case of ore of volcanicorigin, simple crushing can be provided to a grain size of around 150 to500 μm.

In the digestion tank 2, the dissolution of calcium phosphate is rapidand intense, in the form of H₃PO₄ and soluble MCP. The cloudy liquidobtained is sent through the pipe 4 to a separation device 5, forexample a filter press where the undissolved solids are separated at 6,after the advantageous addition of a suitable filtration adjuvant, knownper se, and removed.

The liquid phase issuing from the separation contains, in dissolvedform, phosphoric acid, monocalcium phosphate MCP, calcium chloride and afew residual impurities. The yield of the attack is greater than 80%,preferably 90% and very advantageously 95% by weight, expressed as P₂O₅,and the temperature is ambient temperature.

The liquid phase separated from the attack liquor is then transferredthrough the pipe 7 into a neutralization tank 8, where the dicalciumphosphate DCP is precipitated by introducing, into the liquid phase, at9, calcium derivatives which increase the pH, such as calcium carbonateor milk of lime.

In order to separate the precipitate, it is possible for example todischarge the neutralised liquid phase at 10 and to cause it to passover a belt filter 11 where the solid material is separated, that is tosay a moist cake of DCP 12, containing approximately 40-50% by weightP₂O₅, analysed on dry product, 25-28% Ca and possible traces ofimpurities. The filtrate is discharged at 13. It consists of an aqueoussolution of CaCl₂, easily eliminated, since it is almost non-polluting,and even easily reprocessable. The calcium chloride can be used forexample as an antifreeze product on roadways.

It is also possible to make provision for directing this aqueoussolution of very pure CaCl₂ through a pipe 41 to a reactor 42, which isfed by the conduit 43 with an aqueous solution of sulphuric acid. Inthis reactor, stirred at a temperature of approximately 60° C., for onehour, insoluble calcium sulphate is formed, which precipitates in a verypure form. Through the pipe 44, the liquor resulting from the sulphuricattack is subjected to a separation step, for example by filtration at45. The solid phase formed of calcium sulphate is discharged at 46 andthe liquid phase formed of an aqueous solution of very pure HCl isreturned, through the conduit 47, to the dilute hydrochloric acid supplypipe 3.

As sulphuric acid is more common and more available in large quantitiesthan hydrochloric acid, this recycling further improves the yield of thehydrochloric attack and even makes it possible to provide this in placeswhere HCl is difficult to procure.

In the module illustrated in FIG. 2, it will then be possible to producepure phosphoric acid by a liquid-liquid extraction. For this purpose,the DCP salt, for example obtained in the form of a filtration cake at12 at the belt filter 11, is brought at 14 into a solubilization tank15, where it is solubilised by a new hydrochloric acid attack introducedat 16. However, this step is performed this time with a moreconcentrated solution of HCl, for example 15% to 20% by weight. Thissolubilization makes it possible to obtain an aqueous solutioncontaining phosphoric acid mixed with CaCl₂, a soluble salt of calciumchloride. This solution has a P₂O₅ concentration of around 10% to 12% byweight, which will positively improve transfers at the subsequentliquid-liquid extraction operations and will give rise to concentratedphosphoric acid solutions at the time of re-extraction.

The elimination of the insoluble matter takes place during the attack inthe tank 2, which in the liquid-liquid extraction avoids the problems ofthe formation of emulsions during treatments of the aqueous phase withan organic solvent. The use of purified DCP enriched with P₂O₅ for thepreparation of the initial aqueous phase of the extraction thus affordsmore flexibility in the choice of the ore and reduces the number ofextraction stages to be provided.

The solubilised aqueous phase is then sent through the pipe 38 into anextraction column 17, into which an organic solvent, for examplen-butanol, by way of extraction agent, is fed in reverse flow throughthe pipe 18. The organic solvent selectively extracts the P₂O₅ from theaqueous phase and entrains it through the pipe 39 to a washing column19, and then through the pipe 20 into a re-extraction column 21.

In the re-extraction column 21, the organic phase is put in contact withwater fed in reverse flow by means of the pipe 22. The water extractsthe P₂O₅ from the organic phase. The aqueous phase obtained leaves thecolumn 21 through the pipe 23 and is distributed partly to aconcentration device 24, through the pipe 25, and partly to the washingcolumn 19, through the pipe 26. There the aqueous phase containing P₂O₅serves for washing the organic phase in reverse flow, for eliminatingthe chloride content and the last impurities, not eliminated in themodule illustrated in FIG. 1, and is then once again transferred intothe solubilization tank 15 by means of the pipe 27.

The organic phase coming from the re-extraction is recycled through thepipe 28 to the extraction column 17.

The concentration device 24 may be a normal triple-effect evaporationinstallation which successively vaporises the traces of solvent, whichare recycled to the extraction column 17 through the pipe 29, the HClstill contained in solution, which is recycled to the feed pipe 16 forthe solubilization of DCP through the pipe 30, and a large amount ofwater, at 31. Phosphoric acid is harvested in the form of a concentratedpurified solution, through the pipe 33.

The aqueous phase issuing from the extraction column 17 is thentransferred through the pipe 34 to a steam entrainment column 35. Steamis introduced at 36 into this column and entrains the organic solventload present in the aqueous phase. The organic solvent, entrained bysteam, is recycled into the extraction column 17 through the pipe 37.The aqueous solution of CaCl₂, which may possibly still be treated inorder to eliminate a few heavy metal impurities from it, can in its turnbe discharged at 40.

As can be seen, only some of the CaCl₂ resulting from the introductionof HCl in the method must be treated by a steam entrainment, whichgreatly reduces the energy cost, compared with the methods which effecta liquid-liquid extraction directly on the liquid phase separated fromthe liquor resulting from the hydrochloric attack.

The invention will now be described in more detail by means of exampleembodiments, given non-limitingly.

EXAMPLE 1

Phosphate ore of Moroccan origin which has a P₂O₅ content of 33% and atotal CaO content of 50% (CaO representing all the Ca available in theore as Ca₃(PO₄)₂, CaCO₃, CaO and other Ca compounds) is fed at a rate of15 kg/h into a stirred reactor in order to be attacked there by a 5% byweight solution of hydrochloric acid. The feed rate of the HCl solutionis 141 litres/h, which gives an HCl/Ca normalized ratio of 0.7, i.e.,1.4 mol HCl per mol Ca.

CaO present in the ore is: 50%×15 kg=7.5 kg CaO=0.1339 kmole Ca

P₂O₅ present in the ore is: 33%×15 kg=4.95 kg P₂O₅.

As the density of the HCl solution is approximately 1.15, the mass ofadded HCl is 141×1.15=163 kg.

The non-viscous attack liquor is then transferred into a filter pressafter intermediate passage through a buffer tank (residence time in thelatter approximately 1 hour). The temperature remains ambient. Theinsoluble matter is separated in the filter press.

The solution emerging from the filter press is routed at a rate of 188litres/hour. The content of P₂O₅ is 2.76%. The amount of P₂O₅ in theclear liquor is: 0.0276×170.5=4.706 kg P₂O₅. Hence, the yield of attackis equal to: 4.706/4.95=95%.

On analyzing the Ca content and the Cl content of this clear liquor, itis possible to calculate the free acidity, i.e., the fraction of P₂O₅ inthe clear liquor which is present under the form of H₃PO₄. From thiscalculation a free acidity of 81% is obtained.

The solution emerging from the filter press is then conducted to twoneutralization reactors disposed in series. Calcium carbonate issupplied there at a rate of 5.8 kg/hour, which gives a Ca/P molar ratioof 1. The residence time is 100 minutes over both of the two reactors.

After reaction and formation of DCP, a small quantity of milk of lime isadded to the pulp at the second neutralization reactor in order toexhaust the phosphate load. The concentration of the milk of lime is 245g of Ca(OH)₂/kg and the feed rate into the second reactor is 3litres/hour.

The neutralised pulp is then directed to a belt filter. The cakeobtained consists principally of DCP.

The addition of Ca(OH)₂ is calculated in order to obtain a Ca/P molarratio in the pulp of 0.2. The pH in the filtered mother liquors isapproximately 5 and the residual quantity of phosphate therein is lessthan 1 g/kg.

EXAMPLE 2

The conditions of Example 1 are repeated but in the neutralizationreactors only calcium carbonate is added at a rate of 7.1 kg/hour, whichgives a Ca/P molar ratio of 1.2.

The following table illustrates the composition of the various productsobtained during a process as applied in Example 1.

TABLE Moisture P₂O₅ Ca Cl Na K Mg F SO₄ Si Fe (%) (% m/m) (mg/kg)(mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) As(mg/kg) MCP filtrate  2.76  28365 47735  124 — 50 50 50 — 22.0 0.68Insolubles cake 33.92 total 13.52 325430 16835 2896 585 545 207384 122369200 3700 12 soluble  3.31  36327 37516 DCP cake Dry 36.27 289008  803623.5 584 2.01 Moisture Al Cd Cu Pb Zn Ni Cr (%) (mg/kg) (mg/kg) (mg/kg)(mg/kg) (mg/kg) (mg/kg) (mg/kg) Ti (mg/kg) V (mg/kg) U (mg/kg) MCPfiltrate  4 1.8  0.42 0.31 9.5 3.9 6.2 33.0 3.1 DCP cake Dry 139 0.651.3  3.7  36 3.5 73 7.6 56

EXAMPLE 3a

In a stirred 2 litre reactor a solution of CaCl₂ containing 50% CaCl₂,as obtained as filtered mother liquors at the end of Example 1, is mixedcontinuously at a rate of 4 kg/hour with 96% concentrated sulphuricacid. The feed rate of the sulphuric acid is 0.54 kg/hour.

The residence time in the reactor is 30 minutes.

The pulp formed after the appearance of the gypsum crystals is thentransferred into a Buchner filter. The temperature of the reaction is40° C. The solution emerges from the filter at a rate of 3.36 kg/hourand contains mainly hydrochloric acid (10% HCl). It is recycled at theore attack reactor for a new cycle. The gypsum cake (rate 1.18 kg/hourat 30% moisture) is washed and then discharged. The washing water isrecycled with the filtrate. The cake obtained is mainly calcium sulphatedihydrate.

The addition of sulphuric acid is calculated so as to obtain in the pulpa Ca/H₂SO₄ molar ratio of 1.

Analysis of the hydrochloric acid solution recovered:

SO₄═: 16544 ppm Ca++: 3749 ppm Cl—: 99507 ppm EXAMPLE 3b

In a stirred 2 litre reactor, a solution of CaCl₂ containing 15% CaCl₂is mixed continuously at a rate of 4 kg/hour with 96% concentratedsulphuric acid. The feed rate of the sulphuric acid is 0.48 kg/hour.

The residence time in the reactor is 30 minutes.

The pulp formed after the appearance of the gypsum crystals is thentransferred into a Buchner filter. The temperature of the reaction is40° C. The solution emerging from the filter at a rate of 3.41 kg/h andcontaining mainly hydrochloric acid (10% HCl) is recycled at the oreattack reactor for a new cycle. The gypsum cake (rate of 1.062 kg/hourat 30% moisture) is washed and then discharged. The washing water isrecycled with the filtrate. The cake obtained is mainly calcium sulphatedihydrate.

The addition of the sulphuric acid is calculated so as to obtain in thepulp a Ca/H₂SO₄ molar ratio of 0.09.

Analysis of the hydrochloric acid solution recovered:

SO₄═: 11045 ppm Ca++: 6978 ppm Cl—: 101846 ppm EXAMPLE 4

DCP obtained by the method of Example 1 is allowed to be digested by a20% solution of hydrochloric acid. The dissolution liquor has thefollowing analysis: 20% H₃PO₄, 10% HCl, 10% CaCl₂. This liquor is put incontact, in reverse flow, with an organic solvent, n-butanol, in acolumn at a rate of 100 litres/hour of aqueous phase and with an O/Aratio by volume (organic phase/aqueous phase) of 2. The phosphoric acidand hydrochloric acid pass into the solvent whilst the major part of thecalcium chloride and impurities remain in the aqueous phase (P₂O₅extraction yield of 80%). The organic solvent containing HCl and H₃PO₄(analysis: 10% H₃PO₄, 2.5% HCl, 0.8% CaCl₂) is put in contact in athree-stage extractor with a fraction of the aqueous solution ofphosphoric acid obtained during the following phase (analysis: 30%H₃PO₄) and this with an O/A ratio by volume of 6. This operationeliminates from the organic solution the calcium chloride and theimpurities entrained during the liquid-liquid extraction, which makes itpossible to achieve high purities. The washing solution is then recycledat the head of the extraction by organic solvent. The efficiency ofelimination of the calcium from the organic phase is very high: >99%.

The organic solution is washed in reverse flow with water in a columnwith several plates. The water extracts the phosphoric acid andhydrochloric acid of the organic phase. The organic solvent, with theacids removed, can be recycled at the extraction step. In addition, thephosphoric acid solution passes to a concentration step.

Yield of the operation: 80% re-extraction of P₂O₅. All these operationstake place at ambient temperature.

The solution of dilute phosphoric acid, recovered, is treated byevaporation in order to concentrate the solution and to eliminate anytraces still present of HCl and solvent, by distillation.

It must be understood that the present invention is in no way limited tothe embodiments described above and that many modifications can be madethereto without departing from the scope of the accompanying claims.

1. Method of preparing a phosphoric acid salt, comprising at least oneattack on phosphate ore by an aqueous solution of hydrochloric acidhaving a HCl concentration of no more than 10% by weight, with theformation of an attack liquor, said attack comprising a dissolving ofthe phosphate of the ore in said aqueous solution of hydrochloric acid,with a yield of attack greater than 80% by weight, expressed as P₂O₅,the obtained attack liquor containing this dissolved phosphate in theform of phosphate ions and having a normalized molar ratio of HCl to Caof between 0.6 and 1.3, a first separation, in the attack liquor,between an insoluble solid phase containing impurities and separatedaqueous phase, containing in solution said phosphate ions, chloride ionsand calcium ions, a neutralization of the said separated aqueous phaseby the addition of a calcium compound in order to form, with phosphateions contained in this aqueous phase, a calcium phosphate insoluble inwater, which precipitates, and a second separation, in the saidneutralised aqueous phase, between a liquid phase and a precipitatedsolid phase based on the said calcium phosphate insoluble in water. 2.The method according to claim 1, wherein the HCl concentration is 3% to5% by weight.
 3. The method according to claim 1, wherein the attackstep is performed at ambient temperature.
 4. The method according toclaim 1, wherein the calcium compound of the neutralization step ischosen from amongst a group consisting of calcium hydroxide, calciumoxide and water-soluble calcium salts, such as calcium carbonate, andthe calcium phosphate insoluble in water is calciummonohydrogenophosphate (DCP).
 5. The method according to claim 1,wherein the separated precipitated solid phase, based on the saidcalcium phosphate insoluble in water, has a concentration of 40% to 50%by weight P₂O₅ and 25% to 28% Ca.
 6. The method according to claim 1,comprising a treatment of the liquid phase issuing from the secondseparation and containing calcium ions and chloride ions in solution bymeans of an aqueous solution of sulphuric acid with the formation ofinsoluble calcium sulphate, which precipitates, and an aqueous phasebased on hydrochloric acid, an isolation of the calcium sulphateprecipitate and an at least partial recycling of the aqueous phase basedon hydrochloric acid in order to form the aqueous solution ofhydrochloric acid.